This invention relates generally to devices employing the electrooptic effect and, more particularly, to electrooptical devices for converting the polarization mode of light between the transverse-electric (TE) mode and the transverse-magnetic (TM) mode. The electrooptic effect is a phenomenon in which the application of an electric field results in a change in the refractive index of a waveguide material, and the change in refractive index in turn produces a desired optical effect, such as a change in the polarization mode of light propagating along the waveguide. Electrooptical polarization converters have a wide range of applications in communications systems using optical fibers. The same structure can also be used as a switch or as an optical modulator.
One of the most commonly used materials for devices employing the electrooptic effect is lithium niobate (LiNbo.sub.3). Waveguides are typically formed in lithium niobate by in-diffusion of titanium along a desired waveguide path. This structural combination has good electrooptical properties and is relatively simple to fabricate. However, it also has some serious disadvantages, especially in the context of a polarization mode converter.
Prior work in the area of electrooptic-effect polarization converters is typified by papers by Rod C. Alferness, including "Guided-WaveDevices for Optical Communication," IEEE J. Quant. Elect., VOL. QE-17, No. 6, June, 1981, pp. 946-59; "Electrooptic Guided-Wave Device for General Polarization Transformations," IEEE J. Quant. Elect., VOL. QE-17, No. 6, June, 1981, pp. 965-69; and "Waveguide Electrooptic Modulators," IEEE Trans. on Microwave Theory and Techniques, VOL. MTT-30, No. 8, Aug. 1982, p. 1121. In all of this prior work, the waveguide propagation direction is chosen to be transverse to the optical axis of the lithium niobate crystal, to take advantage of a relatively high coupling coefficient that this provides. As is well known, the electrooptic effect may be defined mathematically by an electrooptic tensor that relates the electric fields in three dimensions to the corresponding induced changes in refractive index. Selection of a propagation direction perpendicular to the optical axis results in electrooptically induced coupling between the TE and TM modes via one of the "off-diagonal" coupling coefficients of the tensor, namely the r.sub.51 coefficient. Although this provides a numerically large coupling coefficient, there are a number of disadvantages inherent in the choice.
First, the material refractive indices "seen" by the two polarization modes are significantly different, regardless of the electrooptic effect. One mode will experience the x-axis or y-axis index, known as the ordinary index n.sub.o, and the other mode will experience the z-axis index, known as the extraordinary index n.sub.e. For efficient coupling between the two modes, there needs to be phase matching between the modes, but when the modes see significantly different refractive indices, a large mismatch in the phase velocities results. In the prior approaches to this problem, periodic electrode structures were needed to satisfy the phase matching conditions. This gave rise to one of the most important disadvantages of the prior approaches. Because of the presence of the electrode structures, the devices behaved as wavelength filters, in which the bandwidth depended on both the period and length of the electrodes. A typical bandwidth of approximately 15 .ANG. has been reported by Alferness, for a device with a 6 mm. long electrode operating at a wavelength of 0.6 micron.
Another difficulty relating to the prior devices is that the difference in refractive indices seen by the two modes is highly temperature sensitive. Therefore, even if the resultant phase mismatch can be compensated by appropriate electrode structures, this solution will be effective only over a narrow temperature range.
Even more important considerations arise from two major problems that arise from the use of lithium niobate in electrooptical devices. One problem relates to optical damage due to the photorefractive effect, and the other relates to out-diffusion of lithium oxide. In the photorefractive effect, an impurity, such as a ferrous ion (Fe.sup.2+), in the waveguide will absorb a photon of light and thereby generate free charge carriers in the form of an electron-hole pair. The charge carriers will move along the optical axis of the material, which is perpendicular to the waveguide propagation direction, under an internal electric field along this axis of the crystal. These mobile charge carriers accumulate on opposite sides of the waveguide, or on the top and bottom regions of the waveguide, depending on the orientation of the optical axis. The resulting electric field induces an unwanted refractive index change through the electrooptic effect. As the optical power transmitted through the device is increased, the photorefractive effect also increases. In practice, the photorefractive effect imposes an upper power limit on the operation of the waveguide, beyond which the waveguide tends to operate less effectively, or may suffer long-term optical damage. Inherently, the effect also limits operation to longer wavelengths, since shorter wavelengths have higher energies and increase the likelihood of optical damage.
The other important disadvantage of lithium niobate is that there tends to be an out-diffusion of lithium oxide (Li.sub.2 O) during the process of in-diffusion of titanium. This results in the formation of an upper layer on the lithium niobate that no longer has the required refractive index properties. This layer tends to scatter light horizontally out from the waveguide, in the z-axis direction, which is perpendicular to the waveguide. This effect is more pronounced when longer diffusion times are used, to ensure a deep diffusion of the titanium. Unfortunately, however, a deep diffusion is typically required to ensure good coupling between the waveguide and optical fibers used in conjunction with the device. Design of a waveguide addressing this problem involves a trade-off between selecting a deep diffusion to minimize coupling losses with fibers, and selecting a shallow diffusion to minimize losses in the waveguide itself due to out-diffusion of lithium oxide. Prior to the present invention, there appears to be no good compromise.
It will be appreciated from the foregoing that there is still a need for innovation in the design of electrooptical devices such as mode converters. In particular, there is a need for a polarization mode converter that is wavelength independent, and is immune to the problems of optical damage temperature sensitivity, and out-diffusion of lithium oxide. The present invention provides the desired solution to these problems.